Germany had a war going on with the Soviets at the same time you know. They had plenty of rifles. They just bit off more than they could chew. At the start of the war they had plenty of manufacturing capability. They just lost a lot of it because we could bomb the crap out of them. That's why we could build rifles faster. They couldn't touch us. We could bomb the crap out of them. So air superiority could be said to be the reason we won the war. We shouldn't forget that much of what we built is at the bottom of the Atlantic because it didn't make it to Europe. So it could be said that controlling the air space in the Battle Of The Atlantic won the war for us because that's what doomed the U-boats and the damage they did to our convoys. Without that advantage we could have lost far more of those rifles we built. And if Hitler had succeeded in capturing England and North Africa chances are we would not have been able to gain a foothold in Europe so it could be said that Patton won the war or that the Hurricanes and Spitfires won the war. It could certainly be said that breaking the German code was the reason we won the war because it made all the difference at El Alamien in N. Africa and it allowed the Allies to be ready for German air attacks on England. Or possibly the brave Brit pilots who rammed their fighters into the big German bombers just as they parachuted out of their planes was the key moment in winning the war because when the Germans saw that they knew they could never win the Battle Of Britain.

There were many, many reasons that could be said were the factor that made the difference. Of all the choices I'd say the breaking of the code was the biggest reason we won the war. It allowed us to win the big battles when we really needed to win. It put the Bismarck in range of the Allies, for example, which allowed that lucky shot that jammed her rudder allowing the heavy British battleships to finish it off. That could have made the difference in the Battle Of The Atlantic too. It also led to the Tirpitz being kept in a dock in Norway so it wouldn't be sunk too. Yes it was a constant threat and tied up a lot of naval power but it never fired a shot at the Allies so it wasn't really much of a factor despite being just as big as the Bismarck.

I just don't think it's so easy to narrow down the reasons for victory to one thing. And, again, if I was to choose one reason it would be the Ultra intercepts.